1. Field of the Invention
The present invention relates to sensor systems for detecting recording media. In particular, the present invention relates to a sensor system for detecting a recording medium in an image forming apparatus and an apparatus that uses the sensor system and that identifies a recording medium.
2. Description of the Related Art
Conventionally, some image forming apparatuses for forming toner images on recording media by an electrophotography process incorporate picture reading sensors for identifying recording media. In such an image forming apparatus, various types of recording media are used. In order to sufficiently fuse and fix an image for all types of recording media, identifying a recording medium and switching to a fixing condition suitable for the identified recording medium before performing a fixing process are necessary.
Examples of the method for identifying a recording medium include a method in which a user sets the size and kind (hereinafter referred to also as sheet type) of medium on an operation panel of an image forming apparatus and a fixing processing condition is switched depending on the settings. Examples of the fixing processing condition include a fixing temperature and a speed of conveying a recording medium passing through a fixing unit (fuser).
When an overhead transparency (OHT) is used as the recording medium, a transmission sensor disposed within the image forming apparatus automatically detects whether the recording medium is an OHT or not. If light passes through the recording medium, the recording medium is identified to be an OHT; if not, the recording medium is identified to be plain paper other than the OHT. In accordance with this identification, the fixing temperature or the speed of conveying the recording medium is set.
FIG. 8 illustrates the structure of the known recording-medium identification sensor 118. The recording-medium identification sensor 118 includes a light-emitting diode (LED) 1 serving as a light emitting unit, an image pickup element 2 serving as an image reading unit, a condensing lens 3, and an imaging lens 4. A surface of a recording-medium conveying guide 8 or a surface of the recording medium 107 on the recording-medium conveying guide 8 is radiated with light beams emitted from the LED 1 reaches via the condensing lens 3. Light beams reflected from the recording medium 107 are gathered to form an image on the image pickup element 2 via the imaging lens 4. Therefore, a picture of the surface of the recording-medium conveying guide 8 or a picture of the surface of the recording medium 107 is read. In this example, the LED 1 is disposed such that the surface of the recording medium 107 is obliquely radiated with light beams emitted from the LED 1 at a predetermined angle, as shown in FIG. 8.
FIG. 9 illustrates the relationship between pictures of the surfaces of the recording media 107 read by the image pickup element 2 in the recording-medium identification sensor 118 and the pictures of the outputs from the image pickup element 2 that are processed into a digitized form of 8×8 pixels. This digitization is performed by converting an analog output from the image pickup element 2 into, for example, 8-bit pixel data with an analog-to-digital (A/D) converting unit (not shown).
FIG. 9 further illustrates an enlarged picture 50 indicating the surface of a coarse recording medium A whose surface fibers are relatively rough, an enlarged picture 51 indicating the surface of a generally used recording medium B, which is a sheet of plain paper, and an enlarged picture 52 indicating the surface of a glossy recording medium C whose surface fibers are fully compressed. When these enlarged pictures 50 to 52 read by the image pickup element 2 are digitized, the results are shown as pictures 53 to 55 in FIG. 9.
As illustrated in FIG. 9, pictures of surfaces of recording media vary depending on the kind of recording medium. This is mainly because the state of fibers of the surface of a recording medium varies. The picture that is digitized from the picture of the surface of the recording medium read by the image pickup element 2, as previously described, varies with the state of paper fibers of the surface of the recording medium, so that these variations allow identification of the recording medium.
FIG. 10 is a flowchart showing control of a condition for a fixing process using the known recording-medium identification sensor 118. The process flow of FIG. 10 is executed by a control processor included in the color image forming apparatus.
In FIG. 10, the control processor first lights the LED 1 (step S001) and reads a picture of the recording medium 107 by the image pickup element 2 (step S002). This picture reading process is carried out multiple times to read multiple areas on the recording medium 107.
The control processor extinguishes the LED 1 (step S003) and adjusts constants for gain calculation and filter calculation in a gain adjusting unit (not shown) and a filter calculating unit (not shown), respectively, included in the control processor (step S004). These gain and filter calculation processes are performed in accordance with programs stored in a read-only memory (ROM) (not shown) within the control processor.
The gain calculation is performed by, for example, adjusting the gain of an analog output from the image pickup element 2. If the amount of light reflected from the surface of the recording medium 107 is too large or too small, the picture of the surface of the recording medium 107 cannot be sufficiently read and thus the variations in the picture cannot be derived. In this case, the gain is adjusted by the control processor.
For the filter calculation, when an analog output from the image pickup element 2 is converted into 8-bit digital data with 256 levels of gray, calculation processing of 1/32, 1/16, ¼, or the like, is performed. In other words, a noise component of an output from the image pickup element 2 is removed.
The control processor determines whether information about pictures sufficient for the next calculation of picture comparison can be obtained or not (step S005). If it is determined that sufficient picture information can be obtained, the picture comparison calculation described below is performed (step S006), the sheet type is determined on the basis of the result of the picture comparison calculation (step S007), and a fixing temperature corresponding to the determined sheet type is set (step S008).
The control processor controls the temperature in a fixing unit (not shown) in such a way that, if the sheet type denotes a sheet of paper whose surface fibers are coarse, like the recording medium A shown in FIG. 9, the fixing temperature is set high, and if the sheet type denotes a sheet of paper whose surface fibers are smooth, like the recording medium C, the fixing temperature is set low.
A method for performing the picture comparison calculation mentioned above is explained below. In the picture comparison calculation process, a pixel that exhibits a maximum output (Dmax) and a pixel that exhibits a minimum output (Dmin) are derived from the result of reading pictures of multiple areas of the surface of the recording medium 107. This process is performed for every read picture, and the results are subjected to averaging processing.
If the surface has coarse paper fibers, like the recording medium A, a large number of shadows of the fibers are present. As a result, the difference between a bright area and a dark area is large, and Dmax-Dmin is increased. In contrast to this, if the surface has smooth paper fibers, like the recording medium C, shadows of the fibers are small, and thus Dmax-Dmin is reduced. The sheet type of the recording media 107 is determined by this comparison.
Since the control processor needs to perform sampling processing of pictures from the image pickup element 2, the gain calculation processing, and the filter calculation processing in real time, it is desirable that a digital signal processor be used as the control processor.
An image forming apparatus for determining the sheet type of recording media described above is disclosed in, for example, Japanese Patent Laid-Open No. 2002-182518 (corresponding to U.S. Pat. No. 6,668,144).
Since a large number of sheet types of available paper have come into use in recent years, the known image forming apparatus described above has become unable to handle all of the sheet types by using only a detection system of the recording-medium identification sensor. This may cause a condition for fixing processing to be improperly set so that the degree of fixing may be poor. In particular, for an OHT, because a dedicated sheet is present for each printer product, if the condition for fixing processing is not optimized, a resulting image may be not sufficiently fixed or the sheet may be jammed.
In addition, there are various known methods for determining the sheet type of recording media such as thick paper. However, a method using a reflective sensor and a method for mechanically detecting the thickness of a sheet of paper, for example, require a dedicated sensor for detecting the thickness of a sheet of paper. This increases the total cost of ownership for an image forming apparatus, and therefore, it leads to poor cost performance.
As one approach to address the problems described above, in order to identify a recording medium more precisely, in addition to known identification, identification is proposed that uses a unit configured to determine the thickness of recording media, such as thick paper, thin paper, and the like, in accordance with the intensity of transmitted light (the amount of transmitted light) by illuminating the recording media from a side adjacent to the back of the recording medium.
However, since a known identification sensor configured to illuminate the recording media from the side adjacent to the back of the recording medium has a structure in which regular transmitted light directly enters the identification sensor, the identification of an OHT or a sheet of thin paper is largely affected by light emitted from an LED. This degrades the accuracy of identifying the OHT or thin paper.